This invention relates to a method and apparatus for recording digital signals in alternate recording tracks and, more particularly, to a method and apparatus which is particularly adapted for use in a video tape recorder (VTR) in which audio signals are digitized and recorded in the same tracks as the usual video signals.
In a typical VTR, a pair of recording heads are rotatably mounted on a guide drum, such as a rotary guide drum, and are rotated to helically scan successive skewed tracks across a magnetic tape that is transported over the guide drum. In one type of helical scan VTR, the recording heads are diametrically opposed to one another so as to be spaced apart by 180.degree.. As the tape is transported, first one head traces a recording track across the tape and then the other head is positioned to scan a parallel track adjacent to the first track. Thus, the two heads record alternate tracks across the tape.
In the NTSC system, each head records a field of video signals; and the heads rotate at the rate of thirty revolutions per second. Thus, two tracks are recorded during every 360.degree. rotation of the heads. Since the heads rotate at a speed which is greater than the speed at which the tape is transported, the relative head-to-tape speed is sufficient for the high density recording of the video signals. Consequently, even if the tape transport speed is reduced, thereby increasing the overall "recording time" or capacity of a magnetic tape of predetermined length, the recording density remains high.
In order to improve the efficiency at which the recording tape is used, it is desirable to minimize or even eliminate entirely the spacing, or guard bands, between adjacent parallel tracks. In one technique which has been proposed for recording without guard bands, the chrominance component of the video signal is frequency-converted to a relatively low range having a frequency-converted chrominance subcarrier on the order of about 688 KHz, and the luminance component is frequency modulated onto an FM carrier on the order of about 3.5 MHz. The rotary heads which are used to record the alternate tracks have air gaps which exhibit different azimuth angles. Consequently, during a reproducing operation, if a portion of one track that had been recorded by, for example, head A is reproduced by head B, the fact that the reproducing head exhibits a different azimuth angle than was used for recording results in substantial attenuation of the undesired cross talk component that is picked up from the adjacent track. However, this so-called azimuth loss is a function of the frequency of the recorded signal. That is, azimuth loss is more pronounced at higher frequencies than at lower frequencies. Accordingly, although azimuth loss is effective to suppress unwanted cross talk components of the higher frequency FM luminance signal that may be picked up from an adjacent track, it is less effective to suppress unwanted cross talk components in the lower frequency chrominance signal that is picked up from the adjacent track.
Chrominance cross talk components are suppressed by the use of a comb filter in the reproducing circuitry. During recording, the effective chrominance subcarrier frequency in one track differs from the chrominance subcarrier frequency in the next adjacent track. This is achieved, for example, by recording the frequency-converted chrominance component with a subcarrier of constant phase from one line interval to the next in one track, and then reversing the phase at every line interval in the next adjacent track. Upon reproduction, the frequencies of the frequency-converted chrominance components that are picked up from adjacent tracks (i.e. the undesired cross talk components) appear to be interleaved with the frequencies of the frequency-converted chrominance conponents which are reproduced from the track being scanned. By using a comb filter having rejection bands that coincide with the cross talk frequencies, the undesired frequency-converted chrominance components which are picked up from adjacent tracks are attenuated. Thus, color video signals may be recorded and satisfactorily reproduced from adjacent tracks without the need to separate such tracks with guard bands.
In helical scan VTRs of the aforementioned type, the rotary heads are used to record only the video signals in skewed tracks. Audio signals are recorded in a separate track, parallel to the longitudinal edge of the tape and disposed in proximity therewith by a fixed audio recording head. However, if the magnetic tape is transported at a relatively slow speed, the quality of the audio signal which is recorded by the fixed head is not as good as if the tape had been transported at higher speeds.
It is thought that the aforementioned problem of degraded audio signal recording due to the relatively low transport speed of the tape can be avoided, or substantially minimized, if the audio signals are recorded in the very same track as the video signals. For example, a predetermined portion of each track may be dedicated for the recording of audio signals therein. Also, the fidelity with which the audio signals are recorded and reproduced may be improved if such audio signals first are digitized, and the digital audio signals then recorded in the video tracks. However, such recording of the digital audio signals in the video tracks may be faced with the problem of cross talk interference in the event that such tracks are recorded without guard bands. That is, during reproduction, when head A, for example, scans the track that had been recorded either by that same head or a head with an equal azimuth angle, the audio signal that had been recorded in an adjacent track by head B may be reproduced.
As is known, in one typical embodiment of a comb filter, cross talk component rejection is achieved, at least in part, on the recognition of the fact that a video signal, particularly the chrominance component thereof, changes slowly from one horizontal line to the next. That is, the relatively high line-to-line redundancy of video information underlies the implementation of the chrominance component comb filter. Unfortunately, audio signals, and particularly digital audio signals, do not exhibit such line-to-line redundancy. Hence, the typical chrominance component cross talk comb filter would not be successful in rejecting cross talk components of the digital audio signals that are picked up from adjacent tracks.
Also, the digital audio signals generally contain a large number of lower frequency components. These components are not subjected to azimuth loss as are the higher frequency FM luminance components. Accordingly, cross talk rejection of the digital audio signals picked up from adjacent tracks is not expected, even when such cross talk components are reproduced by heads having different azimuth angles than were used to record such components. It is possible, however, to record the digital audio signals as Modified Frequency Modulation (MFM) or Modified Modified Frequency Modulation (M.sup.2 FM) signals, wherein the recording frequencies are high enough to be subjected to azimuth loss. However, when using MFM or M.sup.2 FM recording techniques, both the recording system as well as the reproducing system should exhibit uniform frequency characteristics over the entire frequency range from relatively low to relatively high frequencies. This requirement is, however, quite difficult, especially in VTRs wherein the usual equalizer and amplifier circuits in the reproducing section must be of high precision.